Mechanical seals utilize relatively rotating and contacting seal faces, i.e., mating seal faces of a stator ring fixed to a gland and of a rotor ring fixed to the shaft of a rotary equipment, to isolate and seal a pressurized liquid, i.e., the process fluid, along the equipment shaft. To cool the seals and to aid in preventing any passage of process fluid across the seal faces, a second pressurized liquid, i.e., a barrier fluid, is often introduced to the seals on the rear side of the seal faces opposite that in contact with the process fluid. Typically, vanes are formed on the rotating shaft to accelerate the barrier fluid flowing between the shaft and gland. Springs normally bias the seal faces together.
In balanced seal arrangements the pressurized fluids are also applied to piston areas defined on the rear sides of the seal members opposite the seal faces to aid in closing the seal faces. In general, it is desirable to have the piston area associated with the fluid having the higher pressure to be less than 100% and preferably about 70% of the contact area of the seal faces. This relationship minimizes heat generation from the frictional contact of the seal faces while maintaining a closing force on the seal faces sufficiently high to assure proper sealing. It is also desirable to minimize the contact area of the seal faces so as to minimize heat generation as the seal faces rotate relative to each other. Additionally, when a barrier fluid is employed, a double seal arrangement is often utilized in which the process fluid is confined to one end of the seal and the barrier fluid to the center of the seal with relatively rotating seal faces on either side of the barrier fluid, with the local environment being sealed at the other end of the seal.
In one type of balanced double seal in the prior art both fluids have access to the rear of the respective seal members opposite the seal faces and the desired balance ratio of the piston area to the seal face contact area is achieved by providing O-rings slidable in their O-ring grooves behind the respective seal faces of the seal members. The O-rings slide in the grooves to permit application of fluid pressure from the fluid having the highest pressure to the appropriate piston areas on the sides of the seal members opposite the seal faces. Springs biasing the seal faces together are located within the seal on either side of the seal faces and may be exposed to either or both of the process and barrier fluids. This arrangement has significant limitations. First, since the inner and outer diameters of the O-rings define the balance pressure points for the respective fluids, the radial contact dimension of the seal faces must be sufficiently large to account for the thickness of the O-rings. This limits the design of the seal faces for which minimum contact area is desired. Second, if the O-rings do not slide as intended in their grooves, the balance pressures will not be achieved as intended. Additionally, the springs, exposed to the process and barrier fluids, are subject to contamination and corrosion.
It is therefore an object of the present invention to provide a mechanical seal assembly which overcomes the deficiencies of the prior art.
It is another object of the present invention to provide a balanced seal assembly where the balance pressures are not effected by O-ring movement.
It is another object of the present invention to provide a double balanced double seal assembly where the balance pressures are determined by fixed piston areas defined on rear surfaces of the mating primary seal rings.
It is yet another object of the present invention to provide a balanced seal assembly in which the primary seal members remain squared to each other and to the rotating shaft regardless of shaft runout or endplay.
It is a further object of the present invention to provide a seal assembly having a floating flow ring which optimizes acceleration and delivery of barrier fluid within the barrier fluid chamber of the seal assembly regardless of shaft anomalies.